An organic electroluminescence (“electroluminescence” will be occasionally referred to as “EL”, hereinafter) device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used a laminate structure using tris(8-hydroxyquinolinol)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excited particles which are formed by blocking and recombining electrons injected from the cathode can be increased, and that excited particles formed among the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material of the organic EL device, chelate complexes such as tris(8-quinolinolato)aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected (For example, Japanese Unexamined Patent Application Laid-Open Nos. Heisei 8 (1996)-239655 and Heisei 7 (1995)-138561).
It is recently proposed that an organic phosphorescent materials is used in the light emitting layer of an organic EL device in combination with a light emitting material (for example, D. F. O'Brien, M. A. Baldo et al., “Improved energy transfer in electrophosphorescent devices”, Applied Physics Letters, Vol. 74, No. 3, Pages 442 to 444, Jan. 18, 1999; and M. A. Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, Pages 4 to 6, Jul. 5, 1999).
As described above, a great efficiency of light emission is achieved by utilizing an organic phosphorescent material excited to the singlet state and the triplet state in the light emitting layer of an organic EL device. It is considered that singlet excimers and triplet excimers are formed in relative amounts of 1:3 due to the difference in the multiplicity of spin when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing fluorescence alone can be achieved by utilizing a phosphorescent light emitting material.
In the organic EL devices such as those described above, constructions in which layers such as an anode, an organic light emitting layer, an electron transporting layer (a hole blocking layer), an electron injecting layer and a cathode are successively laminated are used so that light emission in the condition excited to the triplet state or from excimers in the triplet state is not quenched (for example, the U.S. Pat. No. 6,097,147, and International Patent Application Published under PCT No. WO01/41512). To organic EL device with the above construction had the following characteristics:
(i) Either a hole blocking layer having larger energy gap than the light emitting layer or a hole blocking layer having greater triplet energy than the light emitting layer was generally used because the excitation state quenched when the electron injecting layer was adhered to the light emitting layer.(ii) With regard to the hole blocking layer, it was possible to improve recombination probability with electrons by restricting the mobility of the holes from the organic light emitting layer and efficiently accumulating the holes in the light emitting layer.(iii) It was necessary to dispose an electron injecting layer having smaller energy gap than the hole blocking layer between the hole blocking layer and the cathode because a direct connection between the hole blocking layer and the cathode metal furiously degrade the property such as lifetime or efficiency.
However, it was found that the conventional constructions for electron injection have problems. Namely, because the hole blocking layer has large energy gap, and because it works with great resistance as an energy barrier for charge injection transport from the other layer, the driving voltage elevated. Further, although many compounds used for the hole blocking layer held favorable hole barrier capability, they tended to deteriorate, and failed to provide an organic EL device with long lifetime.
Japanese Unexamined Patent Application Laid-Open No. 2002-100476 discloses, as a conventional device, an EL device essentially consisting of an electron injecting layer and an adjacent light emitting layer wherein the lowest excited triplet energy level of a host material in the electron injecting layer is higher than the lowest excited triplet energy level of a host material in the light emitting layer. However, in the EL device with increased triplet energy of an electron transporting material, although it avoids quench, the energy gap of electron transporting material becomes extraordinarily great resultantly reaches to 3 eV or greater because singlet energy is generally 0.3 eV or more greater than triplet energy. In this case, there are problems that the energy barrier against the electron injection from cathode becomes so great that the driving voltage elevates, and in the case where the energy barrier is great, a continuation of an electric current injection will induce degradation of the EL device and will make the lifetime of the EL device short as a result.
Further, International Patent Application Published under PCT No. WO01/93642 discloses an organic EL device wherein the host material in the light emitting layer is formed of the electron transporting material. However, there are problems that, because the host material employed for the EL device had an ionization potential of 5.9 eV or more, it fails to inject holes into the host material, prohibiting the host material from transporting holes thereby causing an elevation of the driving voltage.